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Narrator: Behavioral psychologists have
come up with new views,

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not only of animal behavior, 
but of human nature as well.

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And these views all concern a process
that we take for granted: learning.

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Because we are all truly born to learn.

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Ironically, one of the most important figures
in the study of learning, Ivan Pavlov,

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wasn't concerned with the subject at all.

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At least, not at first.

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Pavlov, a noted Russian scientist,

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won the Nobel Prize for Physiology
and Medicine in 1904.

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As this original footage shows, Pavlov 
was initially interested in digestion

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and the action of the salivary glands.

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By diverting the saliva of dogs into test
tubes, he could precisely measure

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if and how much they salivated
during digestion.

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When food was presented,
the dog salivated quickly,

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an inherited salivary reflex.

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But over repeated testings,
a strange thing happened.

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The dog salivated
before contact with the food.

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Just the sight of the food was enough
to stimulate their drooling.

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Then, just seeing the food dish, or even
hearing the footsteps of Pavlov or his assistants,

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was enough to trigger this built-in reflex.

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What was going on to elicit this response?

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Pavlov decided to find out,
by systematically varying the stimuli

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and measuring the dogs' reaction.

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[Metronome clicking]

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Metronomes, lights, and bells
were all used as stimuli,

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and they all worked as stand-ins for
the food.

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What mattered was not the kind of
stimulus that was used,

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but the fact that it reliably signaled
that food was on the way.

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♫ Electronic Music ♫

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Pavlov had discovered a fundamental
type of learning called "Classical Conditioning."

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An original stimulus elicits an automatic
unlearned response.

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Both stimulus and response
happen naturally. They are unconditioned.

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Then a second, neutral stimulus, that never
elicits the unconditional response by itself,

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is introduced just before the presentation
of the original stimulus.

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[bell rings]

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[bell rings]

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[bell rings]

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If the neutral or signaling stimulus is
presented alone,

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and a response occurs, as if 
the original stimulus were still there,

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we say that conditioning has taken place.

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[bell rings]

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The arbitrary neutral stimulus
becomes a conditioned stimulus.

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[bell rings]

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The reverse is also true.

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Pavlov and others studied the extinction
over time of such conditioned responses.

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[bell rings]

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When the subject learns that the conditioned
stimulus no longer signals a desired event,

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[bell rings]

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the acquisition process is reversed
as the learned connection is gradually weakened.

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[bell rings continuously]

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Pavlov's work, and the work of those who
followed him, led to a remarkable conclusion,

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and that is,
any stimulus an organism can perceive

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is capable of eliciting any reaction
the organism is capable of making.

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This means that virtually any sound, sight, or smell
can influence the way our muscles tense or relax,

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our moods fluctuate, or even the way our
attitudes are formed.

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For instance, if I say "Relax," and then do this,

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[gun shot]

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you're going to be startled and upset.

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After five or six pairings of "relax",
[gun shot]

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just saying the word "relax"

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is going to generate a negative response,
rather than its usual learned reaction.